feat(hyperguild): brain pass-rate subcommand

Adds 'hyperguild brain pass-rate <skill> [--window 7d] [--json]'
calling the new /pass-rate endpoint. Human output:
  tdd: 47 / 50 = 94% (window: 7d)
or 'no data (window: 7d)' when pass_rate is null.

PassRateResult mirrors the response envelope; PassRate is *float64
so null is preserved across decode.

cmd/hyperguild/http_test.go

@@ -84,6 +84,40 @@ func TestBrainClient_Write_Success(t *testing.T) {
 	assert.Equal(t, "knowledge/find-h.md", res.Path)
 }
 
+func TestBrainClient_PassRate_Success(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, http.MethodGet, r.Method)
+		assert.Equal(t, "/pass-rate", r.URL.Path)
+		assert.Equal(t, "tdd", r.URL.Query().Get("skill"))
+		assert.Equal(t, "7d", r.URL.Query().Get("window"))
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"skill":"tdd","window":"7d","pass":47,"fail":3,"skip":0,"total":50,"pass_rate":0.94}`))
+	}))
+	defer srv.Close()
+
+	c := &brainClient{baseURL: srv.URL, http: srv.Client()}
+	res, err := c.PassRate(context.Background(), "tdd", "7d")
+	require.NoError(t, err)
+	assert.Equal(t, "tdd", res.Skill)
+	assert.Equal(t, 47, res.Pass)
+	assert.Equal(t, 3, res.Fail)
+	require.NotNil(t, res.PassRate)
+	assert.InDelta(t, 0.94, *res.PassRate, 0.001)
+}
+
+func TestBrainClient_PassRate_NullRate(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"skill":"tdd","window":"7d","pass":0,"fail":0,"skip":0,"total":0,"pass_rate":null}`))
+	}))
+	defer srv.Close()
+
+	c := &brainClient{baseURL: srv.URL, http: srv.Client()}
+	res, err := c.PassRate(context.Background(), "tdd", "7d")
+	require.NoError(t, err)
+	assert.Nil(t, res.PassRate)
+}
+
 func TestNewBrainClient_DefaultURL(t *testing.T) {
 	t.Setenv("BRAIN_URL", "")
 	c := newBrainClient()